almost there using vtk instead of matplotib
Got check image to use vtk reader instead of mpl, exact same answers as before
Merge pull request #147 from charlesdoutriaux/img_diff_from_vtk

Got check image to use vtk reader instead of mpl, exact same answers as ...
reindented test otherwise unmergeable
Merge remote-tracking branch 'devel/img_diff_from_vtk'